The effect of crop load on budbreak influences return bloom in alternate bearing 'Pixie' mandarin
Alternate bearing trees produce a heavy (on) crop followed by a light (off) crop. Whereas it is well documented for citrus that fruit number in the current crop inversely affects flower number in the return bloom, when in the phenology of the tree and how fruit exert an effect on floral intensity the following spring remained unresolved. 'Pixie' mandarin (Citrus reticulata) was used as the model system to investigate when and how fruit perpetuate cyclic differences in floral intensity. Parent shoots (current spring flush shoots) were tagged on on-crop trees and fruit were removed from individual shoots or whole trees. The number of summer and fall (summer/fall) vegetative shoots that developed on parent shoots with and without fruit and the contribution of spring shoots (floral and vegetative) made by parent shoots alone (now 1 year old) and by their summer/fall shoots to return spring bloom was quantified. Removal of fruit from individual shoots on on-crop trees in June or July had no effect on the number of flowers contributed by parent (current spring) shoots to return bloom, but increased total flower number 4-fold because summer/fall shoot number increased more than 8-fold. Removal of fruit from individual shoots of on-crop trees after July had no effect on flower number. In the whole tree experiment, parent + summer/fall shoots of off-crop trees produced more flowers the following spring than on-crop trees due to greater flower production by both parent shoots and their greater number of summer/fall shoots. Removal of all fruit in July from on-crop trees resulted in 2-fold more flowers in spring compared with off-crop trees due to the increased number of flowers contributed by both parent shoots (75% of the total) and the increased number of summer/fall shoots. The importance of summer/fall shoots to return bloom was confirmed by removing all summer/fall shoots from off-crop trees. This reduced floral intensity to that of on-crop trees. Removing all fruit from on-crop trees in December increased the percentage of budbreak in spring and flower number on parent shoots to that of off-crop trees, whereas the number of summer/fall shoots and the number of flowers the parent shoots contributed to bloom were both less than that of off-crop trees. For the branch and whole tree experiments, flower number was significantly correlated with the percentage of spring budbreak on parent + summer/fall shoots (r2 = 0.88, P ≤ 0.0001 and r2 = 0.71, P ≤ 0.0001; respectively). Taken together, the results of this research provide evidence that fruit of the 'Pixie' mandarin reduce floral intensity of the return bloom by inhibiting budbreak, which reduces summer/fall shoot growth and thus the number of nodes that can bear inflorescences and development of spring shoots, which are predominantly floral.